1. Field of the Invention
The present invention relates to an image reader capable of reading images on both sides of a document and an image forming apparatus having the image reader.
2. Related Background Art
Conventionally, an image reader such as a scanner and an image reader provided in a copying machine, a facsimile machine, or the like are each equipped with a document feeder for continuously feeding documents. As this document feeder, there is proposed a document feeder having functions of separating documents laid on a table from one another, feeding the documents one by one, conveying the documents to an image reading portion, and besides, reversing the documents, and the like, as in the case of the invention disclosed in, for example, Japanese Patent Application Laid-open No. 2001-354328.
Various methods of optically reading image information on a document have been proposed in image readers. These methods are broadly classified into two methods. According to one of the methods (hereinafter referred to as fixed-reading), a document is laid on a platen, and image information on the document is read while moving a reading device. According to the other method (hereinafter referred to as flow-reading), a reading device is fixed on a platen at a predetermined location thereof, and image information on a document is read while conveying the document.
In recent years, the image readers often adopt the above-mentioned flow-reading method for reasons of enhanced productivity (an increased processable quantity within a certain period of time) and the like. For instance, an image reader capable of performing fixed-reading is arranged to perform a function of the above-mentioned flow-reading. This image reader is provided with. a platen for flow-reading (a flow-reading range) as well as a platen for having a document laid thereon for fixed-reading (a fixed-reading range).
In addition, with a view to achieving further enhancement of productivity, there is also an image reader adopting a method of performing flow-reading of images on a front side and a reverse side of a document simultaneously by using two reading devices, as in the case of the invention disclosed in, for example, Japanese Patent Application Laid-open No. 2002-120956. This image reader is equipped not only with a reading device for reading an image on a front side of a document laid on a platen but also with a reading device for reading an image on a reverse side of the document on a document feeder side. Moreover, a show-through preventing member, which is intended to prevent show-through of a document, is provided at a position opposed to the reading device on the document feeder side. The show-through preventing member is provided in parallel with the platen. Similarly, another show-through preventing member is disposed also on a side opposed to the reading device on the other side.
However, the image reader provided with the platen for flow-reading as well as the platen for fixed-reading has a problem in that an increase in size of the image reader corresponding to the flow-reading range is inevitable in comparison with the image reader capable of performing only fixed-reading. In other words, while achieving enhancement of productivity based on flow-reading, this image reader has a problem of an increase in size.
The image reader having the show-through preventing member, which is provided at the position opposed to the reading device on the document feeder side, disposed in parallel with and adjacent to the platen has a problem of an increase in size corresponding to the size of the show-through preventing member. In other words, while achieving further enhancement of productivity based on flow-reading, this image reader also has a problem of an increase in size.